EDIT: nevermind... yeah you're right. I must have been thinking of the other way, if you had the mixamp chat outputting into the aux in of the DSS. Because you can do it that way too.... same setup, except the headphones would be plugged into the DSS instead of the mixammp, and the mixamp headphone output would have the 3.5mm cable going to the input of the DSS (the aux in, not the analog in).
So you might want to try it both ways, see what sounds better to you.
So yeah, if you do it the way I first described, I turn the voice knob all the way up, and then you have to kind of ballance the game audio and chat with the volume knobs of both the DSS and the mixamp. Basically I turn the mixamp all the way up, and adjust the DSS up or down as needed, to match the voice volume.